Much human cancer originates in organs not vital to the life and well being of the patient. Examples of such organs are prostrate, breast, ovary, thyroid, and perhaps pancreas and large intestine. Since many of these tumors retain to some extent the morphological and histological characteristics of the organs where they originate, it is possible they retain also antigens peculiar to the organ of origin. If exposed on the surface of tumor cells, such antigens may provide a target for attack on tumor by immunological means. We are looking for similar antigens, in rats, by trying to prepare radioactive antibodies able, after intravenous injection, to localize with high specificity in tumors originating in such non-vital organs. Xenogeneic antisera are prepared in rabbits. Emphasis at the present time is on carcinogen induced and spontaneous mammary tumors originating and carried as transplants in inbred F-344 rats.